A magnetoresistive effect magnetic head is used as a sensor for playing back magnetic information recorded on magnetic recording media on high recording density magnetic recording devices, such as hard disk drives, and is a component of the disk drives which greatly affects the performance of magnetic recording techniques used therein.
Recently, the large magnetoresistive effect has been used in magnetic devices, along with the so-called giant magnetoresistive effect, on multilayer films having ferromagnetic metal layers laminated with a non-magnetic intermediate layer sandwiched in between. In this case, the electrical resistance is changed by the angle formed by the magnetizations of the two ferromagnetic layers sandwiching the non-magnetic intermediate layer. A structure called a spin valve is used such that this giant magnetoresistive effect may be used as a magnetoresistive element. The spin valve usually has a laminated structure of an anti-ferromagnetic layer, a ferromagnetic layer, a non-magnetic intermediate layer, and a ferromagnetic layer, but is not so limited. The magnetization of the ferromagnetic layer in contact with the anti-ferromagnetic layer is essentially pinned by an exchange-coupling magnetic field generated at the boundary surface of the anti-ferromagnetic layer and the ferromagnetic layer.
The output from these devices is generally obtained by freely rotating the magnetization of the other ferromagnetic layer by an external magnetic field. The ferromagnetic layer having magnetization essentially pinned by the anti-ferromagnetic layer is referred to as the pinned layer. The ferromagnetic layer having magnetization which is rotated by the external magnetic field is referred to as the free layer. The playback output is produced as the product of the magnetoresistive (MR) ratio, which is the rate of change in the resistance which depends on the drive voltage and the magnetoresistive effect and the efficiency of the device. The efficiency is an index for indicating how much of the magnetization of the free layer is rotated by the magnetic field applied from the magnetic recording medium. The output increases as the efficiency increases. However, if the output is too large, the change in resistance with respect to the magnetic field becomes nonlinear. Because the performance of a magnetic recording/playback device degrades with nonlinear magnetic fields, normally, the efficiency is set to approximately 20% to 30%. The magnitude of the efficiency is usually controlled to be appropriate by optimizing the materials and the film thicknesses of the magnetic domain control layers provided on both sides in the track direction of the multilayered film of the magnetoresistive effect head.
The current-in-plane (CIP)-giant magnetoresistive (GMR) head using the flow of current in the in-plane direction of a laminated film has been adopted in a spin valve using the magnetoresistive effect. Today, the transition continues toward a tunneling magnetoresistive (TMR) head or a current-perpendicular-to-the-plane (CPP)-GMR head which use the flow of current in the direction of the film thickness of the laminated film. These magnetoresistive effect films are structures which were developed with the objective of improving the signal-to-noise ratio (SNR) of a magnetic head. To increase the playback output, usually, the magnetoresistive (MR) ratio is increased. Currently, the TMR head, which has the highest MR ratio, is used widely for this reason.
Some of the known types of noise include Johnson noise and shot noise originating in the resistance, and Barkhausen noise generated in the magnetic domain of the free layer. Because Johnson noise and shot noise depend on the resistance, an effective way to reduce these types of noise is to reduce the head resistance. Development in this area is progressing with magnetoresistive effect films realizing a high MR ratio and a low electrical resistance. Barkhausen noise is generated when the free layer magnetization has magnetic domains. The generation of magnetic domains can be suppressed by a magnetic domain control layer provided in the magnetoresistive effect head.
However, in the past few years, in addition to the noise sources described above, a noise (mag-noise) generated by the thermal vibrations in the magnetization of the free layer has clearly become a noise problem in devices using magnetoresistive effect heads. Mag-noise (Nmag) can be theoretically calculated from Equation 1, below.
                              N          mag                =                                            Δ              ⁢                                                          ⁢              R                                      H              stiff                                ⁢                                                    4                ⁢                                  k                  B                                ⁢                T                ⁢                                                                  ⁢                α                                                              μ                  0                                ⁢                                  M                  s                                ⁢                V                ⁢                                                                  ⁢                γ                                                                        Equation        ⁢                                  ⁢        1            
Here, ΔR is the maximum change in resistance of a magnetic sensor, Hstiff is the effective magnetic field controlling the magnetic domain received by the magnetic sensor, kB is the Boltzmann's constant (1.38×10−23 J/K), T is the element temperature, α is the Gilbert damping constant, μ0 is the magnetic permeability in a vacuum, Ms is the saturated magnetization of the free layer, V is the volume of the free layer, and γ is the gyromagnetic constant (2.78×103 in/As). This equation is based on the assumption that the magnetization in the entire free layer vibrates uniformly. In practice, the film characteristics of a magnetoresistive effect film are nearly uniform in the film surface, and Equation 1 therefore holds.
The features of mag-noise are the inversely proportional relationship to the square' root of the volume of the free layer and the proportional relationship to the playback output. The first feature means that when the magnetic sensor, and therefore the volume of the free layer, decreases as the recording density increases, the mag-noise essentially increases. The second feature means that the head SNR saturates at some maximum value because the mag-noise increases proportionally when the playback output increases.
Recently, the MR ratio of a CPP-magnetoresistive effect head, represented by a TMR-head installed in a magnetic recording and playback device, has improved noticeably, but simultaneously the mag-noise has increased sharply. Therefore, the head SNR approaches the saturation value predicted theoretically. In other words, the mag-noise was not a problem in conventional CIP-magnetoresistive effect heads because the MR ratio did not increase. The important issue in order to improve the head SNR was to improve the MR ratio when dealing with CIP heads. In contrast, in recent CPP-magnetoresistive effect heads having a high MR ratio, the most important issue is to reduce the mag-noise without lowering the playback output.
However, mag-noise is extremely difficult to reduce without lowering the playback output. For example, since the mag-noise is inversely proportional to Hstiff from Equation 1, the mag-noise decreases when the magnetic field controlling the magnetic domain strengthens. However, because the efficiency is inversely proportional to Hstiff, the playback output decreases in proportion to the mag-noise, and the head SNR does not improve. For the same reason, even if the film thickness or the Ms of the free layer is increased, the mag-noise decreases, but the head SNR does not improve because the efficiency decreases. Thus, because the mag-noise and the efficiency essentially have a trade-off relationship, an effective means for lowering the mag-noise without lowering the playback output would be very beneficial to the field of making and using magnetic heads using current magnetoresistive technologies.